Shares of General Motors Corp (GM.N: Quote, Profile, Research, Stock Buzz) tumbled 24 percent to 62-year lows on Monday after analysts downgraded the automaker, citing cash levels that may fall below the minimum needed in the first quarter of 2009.

Analysts including Barclays Capital to Credit Suisse also warned that while government aid would decrease the risk of a bankruptcy for the No.1 U.S. automaker, any assistance would come at a significant cost to existing shareholders.

GM and Ford Motor Co (F.N: Quote, Profile, Research, Stock Buzz) reported far deeper-than-expected quarterly losses on Friday and said their rate of cash burn had accelerated, as an extended slump in car sales raised questions about the future of the U.S. auto industry.